The present invention generally relates to a compact optical pickup head, and more particularly to an optical pickup head for recording and reproducing information on and from an optical disc, such as a CD or a DVD, in which no diffractive element is used.
It is known that optical pickup heads are used to record and reproduce information such as video or audio data on and from optical data recording media. In a conventional pickup head, the laser diode and the photo detector are installed apart. The optical path of the pickup head is mainly designed with a polarized beam splitter (PBS) installed therein. The polarized beam splitter separates the returning laser beam, which carries data information from the data surface of the recording media, from the same path in reverse of the source laser beam, and guides the returning beam to the photo detector for converting into corresponding electrical signals. The polarized beam splitter makes the pickup head a larger size.
A compact pickup head of related art uses a laser diode and a photo detector incorporated in a single package. The laser diode and the photo detector are almost installed in a same plane and left only with a little distance less than 1 mm apart. The laser diode and photo detector package and some other optical elements constitute a compact pickup head which has a size smaller than conventional pickup heads. The conventional compact pickup head as shown in FIG. 1 uses a holographic optical element (HOE) 91 to process the laser beam. The laser beam emitted from a laser source 90 is split by the holographic optical element 91 into three beams which are the 0 order beam AA, +1 order beam AB and xe2x88x921 order beam AC. Only one (generally the beam AA) of them is guided through a collimator lens 92, a reflection mirror 93 and an objective lens 94 to the optical recording medium 95. The reflective laser beam carrying data information from the data surface of the recording medium 95 passes reversely through the objective lens 94, the reflection mirror 93 and the collimator lens 92 to the holographic optical element 91, and is further split into the 0 order beam AE, +1 order beam AD and xe2x88x921 order beam AF. By the function of the holographic optical element 91, only one of them (for example, the xe2x88x921 order beam AF) is guided to the photo detector 96 for converting optical information into corresponding electrical signals.
But the holographic optical element 91 will waste a lot of the laser power. Supposing the holographic optical element 91 will split a laser source of intensity 1 into 0 order beam of intensity T, +1 order beam of intensity (1xe2x88x92T)/2 and xe2x88x921 order beam of intensity (1xe2x88x92T)/2, then the intensity I of the returning beam toward the photo detector can be calculated as I=T(1xe2x88x92T)/2. To maximize the intensity I, the intensity T of 0 order beam has to be T=xc2xd, and the maximum intensity I is I=xe2x85x9. Therefore, using the holographic optical element will waste at least xe2x85x9e of the laser power. The decrease of the returning laser power causes difficulty in signal transformation and lowers the reliability of data retrieval.
The primary objective of the present invention is therefore to provide a compact optical pickup head which doesn""t utilize a holographic optical element.
Another objective of the present invention is to provide a compact optical pickup head in which the power of laser ray is fully utilized to increase the reliability of signal transformation.
According to the present invention, a birefringent prism and a quarter-wave (xc2xc xcex) plate are disposed in the optical path of a compact pickup head. The birefringent prism is consisted of two crystals made from a birefringent material, and having their optical axes perpendicular to each other. The birefringent prism can be chosen from a Rochon, a Sernarmont or a Wollaston prism which has different refractive indexes for polarized beams in different polarization direction. Therefore, the laser source beam will not be refracted when passing through the birefringent prism (in case of a Rochon or a Sernarmont prism) or will be refracted to a direction differing from the returning beam (in case of a Wollaston prism), while the returning beam is refracted by the birefringent prism to the photo detector. So, the laser power will not be wasted, a higher optical signal can be obtained, and the interference of the returning beam to the laser source is also be prevented.